1. Field of the Invention
This invention relates to an apparatus for production of carbon fibrous materials, a process for production of carbon fibrous materials, a device for preventing the deposition of carbon fibrous materials, and the carbon fibrous materials produced by the process, apparatus and device. More particularly, this invention relates to an apparatus for production of carbon fibrous materials, which has a furnace of tubular reactor such as a vertical furnace of tubular reactor having such a structure that the reactor can hardly be blocked in the inside thereof, a process for production of the carbon fibrous materials using the apparatus, a device for preventing the deposition of the carbon fibrous materials on the inside surface of the furnace of tubular reactor such as a vertical furnace of tubular reactor, and the carbon fibrous materials produced by the apparatus, device and process.
2. Description of Related Art
As an apparatus for production of vapor-phase growth carbon fibers there has hitherto been known an apparatus having a furnace of tubular reactor such as a vertical furnace of tubular reactor.
This apparatus is provided at the upper portion of the vertical furnace of tubular reactor thereof with a feedstock-supplying means for introducing a carrier gas, a gaseous metal catalyst source including a metal catalyst and a gaseous hydrocarbon as a carbon source into the furnace of tubular reactor, a gas-flow rectifying means for rectifying the gas supplied by the feedstock-supplying means to allow the gas to flow downwardly through the furnace of tubular reactor, and a heating means for heating the inside of the furnace of tubular reactor, which is placed to surround the furnace of tubular reactor.
In the prior art apparatus as mentioned above, the gaseous metal catalyst source and hydrocarbon gas are introduced into the heated vertical furnace of tubular reactor together with the carrier gas. The introduced gases are rectified by the gas-flow rectifying means to allow them to pass through the furnace of tubular reactor. The carbon fibers are grown in the heated furnace of tubular reactor.
There have been provided some theories on the mechanism through which carbon fibers are produced in the furnace of tubular reactor. According to one of the theories, a compound for the metal catalyst source introduced in the furnace of tubular reactor is decomposed to form the metal catalyst, and simultaneously the carbon source is decomposed, to thereby produce the carbon fibers. According to another theory, the metal catalyst source is decomposed in the furnace of tubular reactor to form molten metal droplets, into contact with which is then brought the carbon source to result in the decomposition of the carbon source. The resulting carbon is longitudinally grown up with the metal being a core to produce the carbon fibers.
Apart from the mechanism, the resulting carbon fibers are entrained on the streamline flow rectified to pass downwardly through the furnace of tubular reactor, and reach, with help of the carrier gas, the carbon fibers-collecting means such as a means called as a carbon fibers-collector or trapping box via the lower opening of the furnace of tubular reactor.
However, this apparatus as the prior art has the following problem.
This problem is such that, when the carbon fibers are produced in a vapor phase with a molten metal as nuclei formed in the furnace of tubular reactor, fibrous products are deposited on the inside surface of the furnace of tubular reactor.
Some theories on the causes for the deposition of the fibrous products on the inside surface of the furnace of tubular reactor may be considered. For example, according to the first theory, the metal catalyst source is decomposed to form the molten metal, which is then deposited on the inside surface of the furnace of tubular reactor and becomes nuclei, on which the fibrous products are formed by a so-called “growth on substrates”. According to the second theory, the metal catalyst source is deposited and then decomposed on the inside surface of the furnace of tubular reactor to form the metal as nuclei, on which the fibrous products are formed by the growth on substrates. According to the third theory, the carbon fibers produced in a vapor phase in the furnace of tubular reactor are deposited on the inside surface of the furnace of tubular reactor and then grown up longitudinally and/or radially. According to the fourth theory, the above-mentioned theories are combined.
Apart from these theories, once the fibrous products are formed on the inside surface of the furnace of tubular reactor, the carbon fibers formed in vapor phase and falling down through the reactor are deposited on the fibrous products, thus forming an increased amount of the fibrous products different in length and thickness, which finally block the reactor. The blocking of the reactor forces an operator to interrupt the production of the carbon fibers and clean the inside of the reactor, which is very inconvenient to industrial operation.
Furthermore, a pyrolytic carbon layer is formed in the fibrous products deposited on the inside surface of the reactor and the carbon fibers deposited on the fibrous products, and causes the physical properties of the carbon to be inferior as the diameter of the fibers gets greater.
The “vapor-phase growth carbon fibers” used herein means carbon fibers grown in a vapor phase from a compound as a carbon source in the presence of ultrafine particulate nuclei of a metal such as a transition metal. Therefore, the vapor-phase growth carbon fibers contain the ultrafine particulate nuclei of a metal such as a transition metal at the end thereof and are hollow. The graphite network planes of the carbon fibers are stacked like growth rings of wood with the c-axis thereof being rectangular to the axis of the fibers. In other words, the carbon fibers have the graphite network planes stacked to be parallel to the axis of the carbon fibers. The vapor-phase growth carbon fibers may contain fibers generally called as carbon nanotubes or carbon nanofibers. The carbon nanotubes cannot clearly be distinguished from the carbon nanofibers. It is often roughly said that the carbon nanotubes have a diameter of one to smaller than twenty of nanometer, and the carbon nanofibers have a diameter of several tens to one hundred of nanometer. Furthermore, the vapor-phase growth carbon fibers may include hollow fibers having a diameter of smaller than one hundred nm, which were produced at so low a temperature that the metal catalyst particles cannot be molten, with the graphite network planes being conically stacked at an angle of several tens of degree against the fiber axis, and fibrous products of a peculiar shape such as a plate or ribbon shape having a cross-section of smaller than one hundred nm in a longer side with the graphite planes being almost rectangular to the fiber axis.
The above-mentioned problem is more easily caused in a horizontal furnace of tubular reactor, in which many individual flows by convection are more easily produced at the different sites of the reactor, than in the vertical furnace of tubular reactor. In order to overcome this problem, the carrier gas was allowed to flow through the reactor on the inside surface thereof, so that the metal catalyst source, molten metal or carbon fibers cannot be deposited on the inside surface of the reactor. However, the production of the fibrous products on the inside surface of the reactor can be prevented to some extent, but cannot completely be prevented.
As a means for removing the fibrous products from the inside surface of the reactor, were proposed such systems that the reactor in which heat-resisting ceramic balls are placed is rotated, and that the fibrous products deposited on the inside surface of the reactor are intermittently scraped off with a means such as a spatula or rake.
The system using the heat-resisting ceramic balls caused not only the turbulence of the streamline gas flowing through the reactor, but also the degradation of the properties of the vapor-phase growth carbon fibers because of deposition in thickness of the fibers deposited on the heat-resisting ceramic balls. That is, there is such a problem that highly crystalline and hollow carbon fibers cannot efficiently be produced.
In the scraping-off system, the intermittent scraping steps cause the deposition in thickness of the fibers deposited on the inside surface of the reactor, and the permanent presence of the scraping means in the reactor causes the turbulence of the streamline gas flow, rather resulting in a new problem that the deposited amount of the fibrous products may increase.
Thus, the prior art apparatus necessitates the periodic removing operation, for example, at an interval of several minutes. Therefore, the apparatus must be stopped in each of the removing steps, so that the continuous and efficient production of carbon fibers cannot be attained with the prior art apparatus.
The prior art apparatus has the other problems as mentioned below. At the end of the furnace of tubular reactor there are provided feedstock-supplying nozzles for introducing the carbon source gas and the metal catalyst source. There occurs a problem that these feedstock-supplying nozzles are heated inside with a heater to such a temperature in the reactor that the carbon source gas and the metal catalyst source may be decomposed, so that both the sources are decomposed in the nozzles to form the decomposition products, which block the nozzles. In order to remove such a problem, i.e., not to cause the temperature of the nozzles to reach such a temperature that the carbon source and the metal catalyst source are decomposed, the prior art apparatus is provided with any nozzle-cooling means. Thus, the feedstock having a lower temperature than the decomposing temperature is introduced into the reactor, so that the feedstock hardly reaches the decomposing temperature rapidly. As a result, the prior art apparatus has such a defect that the desired vapor-phase growth carbon fibers cannot efficiently be produced.
Of the vapor-phase growth carbon fibers, the carbon nanofibers or carbon nanotubes free of the pyrolytic carbon fiber layer have a relatively high degree of graphitization without subjecting to any particular step of graphitization, and a high electric conductive. However, the productivity thereof is low, because they have a very small thickness and do not grow in thickness. Therefore, the improvement of the productivity has been demanded.
The object of this invention is to an apparatus for production of carbon fibrous materials, the furnace of tubular reactor, particularly vertical furnace of tubular reactor of which apparatus is not blocked with carbon fibers, particularly carbon fibrous materials such as carbon nanofibers or carbon nanotubes, thus realizing the efficient, continuous production of the carbon fibrous materials, but nevertheless, not making the whole size of the apparatus larger.
Another object of this invention is to provide an apparatus for production of carbon fibrous materials, which apparatus can be continued to run for a long period of time, because the furnace of tubular reactor, particularly vertical furnace of tubular reactor is inhibited from being blocked as it possibly can.
A further object of this invention is provide a process for efficient, continuous production of carbon fibrous materials, such as vapor-phase growth carbon fibers, particularly carbon nanofibers or carbon nanotubes.
A still further object of this invention is to provide a device for preventing the deposition of carbon fibrous products on the inside surface of the furnace of tubular reactor, particularly vertical furnace of tubular reactor, in producing carbon fibrous materials, such as vapor-phase growth carbon fibers, particularly carbon nanofibers or carbon nanotubes in the furnace of tubular reactor.
An additional object of this invention is to provide vapor-phase growth carbon fibers including carbon nanotubes or carbon nanofibers, which have a diameter of about 100 nm and below, especially about 50 nm and below, and contain a central hollow core along the axis of the fibers, which core portion is surrounded in parallel by one or more layers of hexagonal crystal plane consisting of carbons, which has a shape like growth rings of wood in the cross section.